The embryonic derivation of thymic lymphocytes and of lymphocytes in the peripheral lymphoid tissues has been controversial. The theory currently in vogue is that, during the ontogeny of the thymus gland, thymic lymphocytes are derived from invading, blood-borne hematocytoblastlike cells rather than from epithelial cells of the thymic rudiment itself. This hypothesis has received substantial support from a variety of experimental protocols in birds and mammals, most notably those in which vascular migration pathways of chromosomally labeled cells have been determined in 7-8 day old chick embryos. We are re-investigating the question of thymic versus the extrathymic origin of lymphocytes in the developing thymus and peripheral lymphoid organs in an amphibian system. The embryo of the leopard frog, Rana pipiens, has provided us with a model in which we can identify and chromosomally label the presumptive thymic tissue, remove it at a very early embryonic stage - even before it can be called a thymic rudiment - and transplant it either unilaterally or bilaterally to an unlabeled recipient of the same age. These chimeric embryos can be reared through metamorphosis and determinations can then be made of the relative percentages of donor and host cells that reside in the differentiated transplant as well as populate the peripheral lymphoid tissues of the host. Such procedures, which are necessary to resolve the question of the thymic versus the extrathymic origin of the lymphocytes in the developing thymus, are technically impossible with mammalian embryos. Our preliminary data indicate that thymic lymphocytes do not originate from blood-borne stem cells that migrate into the thymus anlage but rather arise in situ from elements in the thymic rudiment itself. After thymic differentiation, the lymphocytes (or their descendants) leave the thymus and extensively seed the peripheral lymphoid organs.